LT8601 datasheet(21/38 Pages) LINER

21 / 38 page

LT8603

8603f

For more information www.linear.com/LT8603

APPLICATIONS INFORMATION

From this the ripple current can be specified using:

∆IL = χ •IL(MAX) = χ •IOUT(MAX) •

1 – DMAX

where χ in the above equation represents the percent-

age peak-to-peak ripple current in the inductor relative to

IL(MAX). Choosing the inductor ripple current, ∆IL, has a

direct impact on the choice of the inductor value and the

converter’s maximum output current capability. Choosing

smaller values of ∆IL increases the converter’s output cur-

rent capability but requires larger inductors. Choosing

larger values of ∆IL provides faster transient response

and allows the use of smaller inductors but results in

higher input current ripple, greater core losses, and lower

output current capability. In addition, larger values of ∆IL

at high duty cycle may result in sub-harmonic oscillation.

The typical range for χ is 20% to 40% though careful

evaluation of system stability should be made to ensure

adequate design margin. The peak inductor current will be

the average inductor current plus half the ripple current.

The LT8603 current sense comparator has a built-in cur-

rent limit threshold of 50mV across ISP4 and ISN4 so the

peak voltage across the sense resistor should be kept to

no more than 80% of this or 40mV. Therefore, the value

of RSENSE should be:

RSENSE =

0.04

IL(PEAK)

IL(PEAK) can be calculated from:

IL(PEAK) = 1+











 •

IOUT(MAX)

1 – DMAX

The inductor value to achieve the required ripple is given

by following equation:

L =

VBATT MIN

(

)

∆IL • fSW

• DMAX

The inductor should have a saturation current exceeding

the current limit value of:

ILIM =

0.05

RSENSE

Finally, the ESR and magnetic loss of the inductor con-

tribute to overall system power loss. For best efficiency,

an inductor with low ESR and a core rated for the desired

operating frequency should be chosen. The sense resis-

tor should be chosen to have adequate power handling

capability. The maximum power dissipation is given by:

PRSENSE =

0.052

RSENSE

For example, a 4mΩ could dissipate up to 0.625W.

Boost: MOSFET Selection

For the boost configuration, the selected external MOSFET

should have a BVDSS rating exceeding, with margin, the

larger of either the maximum input voltage or the boost

output voltage plus a diode forward voltage. The maxi-

mum input voltage should include careful consideration

of possible transient conditions. In addition, the chosen

MOSFET should be compatible with the LT8603’s nomi-

nal gate drive of 4.6V and have a low value of RDS(ON)

for best efficiency. The current rating for the MOSFET

must be greater than the peak inductor current. Finally,

the maximum gate drive current required by the external

MOSFET should not exceed the 40mA capability of the

LT8603. The current drawn by the gate driver is given by:

IDRIVE = Qg • fsw

where Qg is the MOSFET gate charge and fSW is pro-

grammed switching frequency.

For example, if Qg is 20nC and fSW is 1MHz, the drive

requirement is 20mA.

Boost: Diode Selection

In the boost converter, the rectifier diode, SD1, only con-

ducts when the switch is off. The average diode current

is equal to the output current. The peak current is equal

to the peak inductor current and is given by:

ID PEAK

(

) = 1+











 •

IOUT MAX

(

)

1 – DMAX

LT8601 データシート(PDF) 21 Page - Linear Technology

LT8601 データシート(HTML) 21 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

データシートダウンロード

リンク URL

LT8601 データシート(PDF) 21 Page - Linear Technology

LT8601 データシート(HTML) 21 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

データシート ダウンロード

リンク URL

データシートダウンロード